Cultured mouse fibroblasts which are transformed by RNA viruses, a DNA virus or a chemical agent, all secrete a 39,000 Mr-phosphoglycoprotein (major excreted protein, MEP) in large amounts. Nontransformed murine fibroblasts secrete MEP after treatment with tumor promoters such as TPA or growth factors such as PDGF. Human cells also synthesize MEP, and in the case of cultured human melanocytes, its synthesis appears to be increased by TPA treatment. We have purified MEP, prepared monospecific affinity-purified antisera against it and cloned a cDNA which codes for MEP from Chinese hamster and mouse cells. The protein contains mannose 6-phosphate, the lysosomal recognition marker. It is processed intracellularly in both transformed and nontransformed cells to give two specific lower molecular weight forms, the lowest of which has a predominantly lysosomal localization. Transformation, TPA and PDGF stimulate MEP synthesis by increasing levels of MEP specific mRNA. We are studying this system as a model of regulation of lysosomal protein synthesis, processing and secretion as it is affected by transformation and agents which mimic the transformed state, such as tumor promoters and growth factors.